In order to facilitate the replacement of worn or broken picks, each pick must be removably fitted into a receiving pocket of the associated holder within which it is locked in position by suitable fastening means. The removal of a pick from its pocket is sometimes made difficult by the settling of comminuted matter, such as coal or rock particles, in the narrow clearances necessarily present between the inserted part of the pick--referred to hereinafter as its shank--and the adjacent walls of the receiving pocket. Such penetration of solid particles will occur even if, as is customary, the excavating head is provided with nozzles through which water is emitted in order to precipitate the evolving dust. Thus, forces exerted upon the tool shank not only during cutting but also during extraction of the pick from its holder may deform the holder itself unless care is taken to insure an optimum mode of seating the shank in its pocket and holding it in position.
German utility model No. 78 23 740 discloses a holder for a pick with an outline in the form of an equilateral triangle allowing any one of its three corners to be used as a working edge of its projecting tip while another corner forms part of a shank projecting into the holder pocket. The shank is held in place by a bolt passing through frustoconical sleeves which converge toward each other and are clamped between the bolt head and a nut, the sleeves bearing upon two trapezoidal wedge pieces respectively lying against a trailing flank of the pick and a confronting rear wall of the pocket. The retaining effect of such a clamp is somewhat uncertain on account of the large angle of divergence (60.degree.) of the leading and trailing flanks of the shank and in view of the absence of any positive-acting abutment.
Another arrangement for releasably retaining a pick in an associated tool holder is shown in German utility model No. 81 16 945.0. According to that arrangement, the trailing flank of the tool shank has a spur underreaching a shoulder at the back of the pocket while its leading flank has a concave surface separated by an arcuate gap from a convex surface of a transverse web spanning the sidewalls of the pocket; the two surfaces are curved about different axes so that the gap converges. A similarly converging arcuate wedge piece is forced into the gap and terminates in a bifurcation whose prongs have ends bent around the web to hold the wedge piece in place. Since an extraction of the pick requires an unbending of the prong ends which upon a subsequent rebending would have lost some of their holding strength, not only the pick but also the wedge piece will have to be replaced; also, the bending and unbending operations are difficult to perform underground and the need for having spare wedge pieces available is an inconvenience.
Aside from the necessity of replacing worn picks, it is sometimes desirable to change the angle of attack of a still usable pick which requires a repositioning thereof in its pocket. Such a repositioning is not possible with the arrangement last described; while the triangular pick of the first-mentioned German utility model has three possible positions, the angle of attack remains the same in all instances.